Storage-stable moulding powder based on epoxy resins

ABSTRACT

A moulding powder which is curable with the application of heat and pressure, comprising a mechanical mixture consisting of: a) a powdered epoxy compound containing on average more than one epoxy group in the molecule and having a softening point of above 60° C., b) a powdered aliphatic or N-heterocyclic amine or a powdered amino group-containing adduct based on an aliphatic, cycloaliphatic, araliphatic or N-heterocyclic amine containing on average more than one amino group in the molecule and having a melting point or softening point of above 60° C. and, optionally, c) customary additives for moulding powders, the particles of a) and b) being smaller than 200 μm, is excellently suited for the preparation of coatings on heat-sensitive substrates and is used in particular in powder coating.

The present invention relates to a storage-stable highly reactivemoulding powder based on epoxy resins and aminic hardeners and to theuse of said moulding powder for the preparation of moulded articles orcoatings, preferably for powder coatings.

Until now, the customary method of preparing powder coatings based onepoxy resins was that using extruders. As is described, inter alia, byBecker/Braun in "Kunststoffhandbuch" (Carl Hanser Verlag Munchen Wien1988), Vol. 10, Chapter 13.10 under powder coatings on pages 1047-1058and, in particular, on pages 1051 and 1052, said preparation is carriedout such that the individual components such as epoxy resin, hardenerand the customary additives for powder coatings, typically flow controlagents, pigments, fillers and catalysts, are comminuted, mixed and thenextruded in the temperature range from 70 to 120° C. depending on thecomposition of the mixture and the type of extruders used. Thetemperature must be such that no pre-crosslinking occurs interferingwith the subsequent processing. After extruding, the extrudate is cooledand ground to a particle size of smaller than 100 μm. Incontradistinction to a mechanical mixture, the particles so obtainedtherefore contain all components combined. The application to thesubstrate to be coated is carried out by known methods, typically byelectrostatic powder spraying or by fluidised-bed coating. The appliedpowder coating is then cured in the temperature range from about 130 to240° C. for 10 to 30 minutes. Disadvantages of powder coatings havinglow curing temperatures, e.g. 130° C., are the relatively long curingtimes of over 15 minutes, the poor flow of the powder coating on thesubstrate as well as the limited storage stability of the formulatedpowder coatings.

EP-Patent 0 042 759 discloses a process for colouring a substratesurface by powder coating, which comprises using powder coating systemswhich are curable with the application of heat and pressure. Thedislosed coating powders consisting of epoxy resin and polyester, or ofepoxy resin and polyurethane, are cured at 180° C. in 3 or 10 minutes,respectively, and are poorly suited for coating substrates susceptibleto heat, such as paperboard or wood, and are not reactive enough forcuring at low temperatures.

JP Application Kokai Sho 61-107980 discloses a process for coating wood,wherein the substrate consisting of wood is first coated with a liquidepoxy resin and then with an epoxy resin in powder form before thecoating is cured with the application of heat and pressure. The liquidas well as the solid epoxy resin contain a catalytic hardener, e.g.2-ethyl-4-methylimidazole, so that the storage stability of the coatingsystems used is low.

It has now been found that storage-stable coating powders based on epoxyresins and aminic hardeners, which are rapidly curable with theapplication of heat and pressure, are obtained if a powdered epoxy resinhaving a particle size smaller than 200 μm and a softening point(according to DIN 51920) of above 60° C. is mechanically mixed with apowdered aliphatic or N-heterocyclic amine or a powdered aminogroup-containing adduct based on an aliphatic, cycloaliphatic,araliphatic or N-heterocyclic amine having a particle size of smallerthan 200 μm and a softening point of above 60° C. without extrusion,giving a physical mixture of the components.

Accordingly, the invention relates to a moulding powder which is curablewith the application of heat and pressure, comprising a mechanicalmixture consisting of

a) a powdered epoxy compound containing on average more than one epoxygroup in the molecule and having a softening point of above 60° C.,

b) a powdered aliphatic or N-heterocyclic amine or a powdered aminogroup-containing adduct based on an aliphatic, cycloaliphatic,araliphatic or N-heterocyclic amine containing on average more than oneamino group in the molecule and having a melting point or softeningpoint of above 60° C. and, optionally,

c) customary additives for moulding powders, the particles of a) and b)being smaller than 200 um.

Suitable powdered epoxy compunds a) are all solid epoxy resins customaryin epoxy resin technology having a softening point of above 60° C. Suchepoxy resins are known, inter alia, from DE-OS 28 38 841 and U.S. Pat.No. 4,175,173, and some are commercially available.

Component a) is preferably an aromatic epoxy resin, typically thepolyglycidyl ethers of polyphenols, preferably of bisphenols, or aN-heterocyclic epoxy resin, such as the diglycidyl compound ofhydantoins or the triglycidyl compound of cyanuric acid.

The epoxy resins which are preferably used have an epoxy content of 0.5to 12 equivalents per kg. Those epoxy resins having a softening point ofbelow 60° C. can be converted in known manner into higher molecularweight epoxy resins having higher softening points by advancement,typically by reaction with a substoichiometric amount of a divalentphenol.

Epoxy resins which are particularly preferred have a softening point ofabove 80° C. and are optionally advanced polyglycidyl ethers of2,2-bis(4'-hydroxyphenyl)propane (bisphenol A), of2,2-bis(3',5'-dibromo-4'-hydroxyphenyl)propane (tetrabromobisphenol A),of bis(4-hydroxypehnyl)methane (bisphenol F) and of novolaks,polyglycidyl derivatives of 4,4'-diaminodiphenylmethane, of4,4'-diamonodiphenylsulfone as well as of hydantoins and of2,4,6-trihydroxy-1,3,5-triazine (cyanuric acid), typically triglycidylisocyanurate.

For the preparation of the novel moulding powders it is preferred to usethe polyglycidyl compounds of bisphenol A, tetrabromobisphenol A,bisphenol F or of a novolak such as phenol novolak or cresol novolak, ortriglycidyl isocyanurate. It is also possible to use mixtures of saidpolyglycidyl compounds.

The aliphatic or N-heterocyclic amines having a melting point of above60° C. which are used as component b) in the novel moulding powders arealso known compounds and include mono-, di- and polyamines. Saidcompounds can be piperazine, 1,10-diaminodecane, 1,12-diaminododecane.

The amino group-containing adducts present in the novel mouldingpowders, which are obtained in known manner by reacting a diepoxycompound with a diamine or polyamine in excess of stoichiometricproportion, are preferably those derived from diglycidyl ethers,typically from bisphenol diglycidyl ethers. Aliphatic di- or polyaminesused for the preparation of the amino group-containing adducts aretypically ethylenediamine, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, propane-1,2-diamine, propane-1,3-diamine,hexamethylene-1,6-diamine, dipropylenetriamine or2,2,4-trimethylhexane-1,6-diamine. Arylaliphatic diamine can typicallybe 1,4-bis(aminomethyl)benzene. Suitable cycloaliphatic diamines aretypically bis(aminomethyl)cyclohexane, bis(4-aminocyclohexyl)methane,bis(4-aminocyclohexyl)sulfone, bis(4-aminomethyl)dicyclopentadiene,2,2-bis(4-aminocyclohexyl)propane,bis(4-amino-3-methylcyclohexyl)methane,2,2-bis(4-amino-3-methylcyclohexyl) propane, 1,4-diaminocyclohexane,1,2-diaminocyclohexane, 1,3-diamino-4-methylcyclohexane or3-aminomethyl-3,5,5-trimethylcyclohexylamine (isophoronediamine).N-heterocyclic di- or polyamines can typically be piperazine,N-2-amino-ethylpiperazine or 4-amino-2,2,6,6-tetramethylpiperidine.

Component b) in the novel moulding powders is preferably an aliphaticamine or an amino group-containing adduct.

Component b) in the novel moulding powders is preferably an aminogroup-containing adduct, particularly preferably of a diglycidyl ethercompound of bisphenol A and ethylenediamine, diethylenetriamine, 1,2- or1,4-diaminocyclohexane or isophoronediamine.

Components a) and b) in the novel moulding powders are usually used inapproximately equivalent amounts, i.e. 0.75 to 1.25 equivalents ofactive hydrogen bound to amino nitrogen atoms of component b) per epoxyequivalent of component a).

The novel moulding powders can also contain in the customary fillers andreinforcing materials of epoxy technology. Suitable fillers are forexample: mineralic and fibrous fillers such as quartz powder, fusedsilica, aluminium oxide, glass powder, mica, kaolin, dolomite, graphite,carbon black as well as carbon fibres and textile fibres. Preferredfillers are quartz powder, fused silica, aluminium oxide or dolomite.Suitable reinforcing materials are typically glass fibres or carbonfibres, fibres of natural materials such as wood or jute, or wood chips.

The moulding powders of this invention are preferably used as powdercoatings. In the case of this preferred application, it is possible toadd the customary additives of the paint industry to the novel powdercoatings, typically light stabilisers, colourants and, in particular,degassing agents, flow control agents or pigments such as, preferably,TiO₂.

Flow control agents are typically polyacetals, such as polyvinylbutyral(Movital® B 30H, supplied by Hoechst), polyethylene glycol,polyvinylpyrrolidone, glycerol, acrylic copolymers such as Modaflow® orAcrylron® MFP, supplied by Monsanto and Protex, respectively. Thedegassing agent is preferabyl benzoin.

The novel moulding powders can be prepared by either milling componentsa) and b) separately to particles having a particle size of <200 μm,removing those particles having a particle size of >200 μm by sieving,and then mixing the powdered components a) and b) mechanically in amixing unit such that a physical mixture is obtained, or by mixingcomponents a) and b) mechanically before grinding and sieving themtogether.

The components can be mixed using customary laboratory mixers, typicallyin a ball mill, in a laboratory multipurpose mixer or in a Rhoenradmixer.

The additives c) can be added during the preparation of the novelmoulding powder to component a) or component b) as well as to bothcomponents before or after grinding and sieving.

Where a pigment is used as additive, it is preferably added to componenta) and/or b) before grinding, and the mixture of component a) andpigment or of component b) and pigment is extruded, e.g. in a Bussko-kneader. The ground and sieved extrudate of component a) or componentb) is then used to prepared the physical mixture.

The novel moulding powders can be used for the preparation of mouldedarticles and coatings, the curing of the moulding powders being carriedout with the application of heat and pressure. The novel mouldingpowders are preferably used for the preparation of chip boards.

The moulded articles prepared from the novel moulding powders preferablycontain fillers or reinforcing agents. The novel moulding powders arepreferably used to prepare chip boards or moulded parts by mixing orimpregnating wood chips or fibres of natural materials with the novelmoulding powders and moulding them with the application of heat in amoulding press to chip boards or moulded parts. Accordingly, theinvention also relates to the moulded articles prepared from the novelmoulding powders, in particular chip boards or moulded parts.

The novel moulding powders are preferably used as coating powders, inparticular for coating heat-sensitive substrates such as paper,paperboard, wood, thermoplasts or duroplasts because the novel mouldingpowders are highly reactive and have the advantage of curing rapidlyunder pressure and heat. Accordingly, this invention also relates topowder coatings consisting of the novel moulding powders.

The coatings are prepared by applying the novel powder to the substrateby sieving, sprinkling or by electrostatic powder spraying. The surfaceof the substrate coated with the powder coating is then covered with aheat-resistant foil, or an antisize is applied to the moulding press.The powder coating is crosslinked in a heatable moulding press underslight pressure. Independently of the layer thickness of the powdercoating applied, crosslinking times of less than 3 minutes are usuallyrequired at a temperature of about 130° C. and a pressure of about 10N/cm².

For electrostatic powder spraying it is preferred to use mouldingpowders having a particle size of <100 μm.

If desired, a heatable roll or a heatable metal band can be used forcuring the powder coating, which would also make a continuous coatingprocess possible.

In another of its aspects, this invention relates to the coatingsprepared from the novel moulding powders.

As mentioned at the outset, the novel moulding powders are distinguishedby being, on the one hand, highly reactive, while having a long storagestability at room temperature or up to 40° C. The moulded articles andcoatings prepared from the novel moulding powders have excellentmechanical properties and the coatings have very good adhesive strengthon the substrates.

The following compounds are used in the Examples:

Epoxy resin A: Mixture of 80 g of diglycidyl ether of bisphenol A and 20g of phenol novolak epoxy resin having an epoxy content of 1.9equivalents/kg and a softening point according to DIN 51920 of 95° C.

Epoxy resin B: Diglycidyl ether of bispheol A having an epoxy content of1.7 equivalents/kg and a softening point according to DIN 51920 of80-90° C.

Epoxy resin C: Cresol novolak epoxy resin having an epoxy content of 4.5equivalents/kg and a softening point according to DIN 51920 of 94° C.

Hardener I: Amino group-containing adduct, having an amine content of3.5 equivalents/kg and a softening point according to DIN 51920 of 94°C., which is obtainable by reacting 1000 g of diglycidyl ether resin ofbisphenol A having an epoxy content of 2.1 equivalents/kg with 485 g ofethylenediamine in an inert organic solvent with subsequent removal ofthe solvent and excess amine by distillation.

Hardener II: Amino group-containing adduct, having an amine content of3.5 equivalents/kg and a softening point according to DIN 51920 of 100°C., which is obtained by reacting 1000 g of diglycidyl ether resin ofbisphenol A having an epoxy content of 2.1 equivalents/ kg with 1400 gof 1,2-diaminocyclohexane in an inert organic solvent with subsequentremoval of the solvent and excess amine by distillation.

EXAMPLE 1

Epoxy resin A and hardener I are ground separately by known standardmethods to a fine powder, the coarse components having a particle sizeof more than 100 μm being removed with a sieve. 735 g of powdered epoxyresin A are physically mixed with 265 g of powdered hardener I in alaboratory mixer (type: Rhoenrad mixer). The gelling time of theready-to-use powder at 130° C. is 21 seconds (sec). There is nosignificant change in the gelling time after storing the powder mixturefor 6 months at 40° C.

The powder mixture is applied to a wood surface (beech), previouslymoistened with water, by electrostatic powder spraying. The wood surfacecoated with the powder coating is covered with a heat-resistant foil(Tedlar®, producer: DuPont). The powder coating is crosslinked in amoulding press heated to 130° C. for 2 min, 30 sec at a pressure ofabout 10 N/cm².

A coating film is obtained having the following properties:

film thickness: about 50 μm.

flow (visual assessment): very good (film surface is not veined).

impact strength*), front side (20 cm•kg): very good (coating does notchip).

cross-hatch adhesion: very good (coating does not chip off the wood).

EXAMPLE 2

530 g of epoxy resin B are homogenised with 470 g of titanium dioxide inan extruder (type: ko-kneader, supplied by Buss, Pratteln, CH) at 75° C.Extrudate and hardener are ground separately by known standard methodsto a fine powder. The coarse components having a particle size of morethan 100 μm are removed with a sieve.

1000 g of powdered extrudate are physically mixed with 175 g of powderedhardener I in a laboratory mixer (type: Rhoenrad mixer). The gellingtime of the ready-to-use powder at 130° .C is 35 sec. The powder mixtureis sieved onto a MDF board (MDF=Medium Density Fibreboard, a compressedfibre board of medium density consisting of wood particles andduroplastic binders), and the wood surface coated with the powdercoating is covered with a Tediar® foil. The powder coating iscrosslinked in a moulding press heated to 130° C. for 2 min 30 sec at apressure of about 10 N/cm².

A coating film is obtained having the following properties:

film thickness: about 60 μm

flow (visual assessment): very good (film surface is not veined)

impact strength according to Example 1, on the front side (20 cm•kg):very good (coating does not chip)

cross-hatch adhesion: very good (coating does not chip off the wood)

EXAMPLE 3

In exactly the same manner as in Example 1, a coating powder is preparedby mixing 223 g of epoxy resin C and 192 g of hardener 1. The gellingtime of the ready-to-use powder at 130° C. is 17 sec. The powder mixtureis sieved onto beech and covered with a Tedlar® foil. The powder coatingis crosslinked in a moulding press heated to 130° C. for 90 sec under aslight pressure of 10 N/cm².

The coating film so obtained has the following properties:

film thickness: about 130 μm

flow (visual assessment): very good (film surface is not veined)

impact strength according to Example 1, front side (20 cm•kg): very good(coating does not chip)

acetone test *): not scratchable

EXAMPLE 4

In exactly the same manner as in Example 1, a coating powder is preparedby mixing 534 g of epoxy resin C and 455 g of hardener II. The gellingtime of the ready-to-use powder at 130° C. is 34 sec. The powder mixtureis sieved onto beech and covered with a Tedlar® foil. The powder coatingis crosslinked in a moulding press heated to 130° C. for 3 min at apressure of about 10 N/cm².

A coating film is obtained having the following properties:

film thickness: about 170 μm

flow (visual assessment): very good (film surface is not veined)

impact strength according to Example 1, front side (20 cm•kg): very good(coating does not chip)

acetone test according to Example 3: not scratchable

EXAMPLE 5

Epoxy resin A and hardener I are ground separately by known standardmethods to fine powders, the coarse components of over 100 μm beingremoved with a sieve. The powders are further processed as follows:

735 g of powdered epoxy resin A are physically mixed with 265 g ofpowdered hardener I in a laboratory mixer. The gelling time of theready-to-use powder at 130° C. is 21 sec. To prepare chip boards, 834 gof wood chips having a water content of about 7% by weight are mixedwith 68.2 g of the ready-to-use powder for 20 minutes in a laboratorymixer. Subsequently, 798 g of this powder mixture are sprinkled into asquare wooden mould having an inner surface area of 625 cm² and areprecompressed with a punch to a square moulded article. Upon removal ofthe wooden mould, the precompressed moulded article is automaticallycompressed and fully cured in a moulding press supplied by Bucher GuyerAG, type KHE 50-40-40, for 12 minutes at a press temperature of 160° C.The chip board so obtained has a thickness of 20 mm.

The transverse tensile strength according to DIN 52 365 was tested in 5samples of the chip board, giving an average value of 0.42 N/mm².

What is claimed is:
 1. A process for producing a coating on aheat-sensitive substrate selected from paper, paperboards, wood, chipboards, medium-density fiberboards (MDF-boards), thermoplasts,duroplasts or moulded parts, by using a powder coating compositioncomprising a mechanical mixture consisting ofa) a powdered epoxycompound containing on average more than one epoxy group in the moleculeand having a softening point of above 60° C.; b) a powdered aliphaticamine or a powdered amino group-containing adduct based on an aliphatic,cycloaliphatic, araliphatic or N-heterocyclic amine, said adductcontaining on average more than one amino group in the molecule, whichamine or adduct has a melting point or softening point of above 60° C.;and, optionally, c) customary additives for moulding powders, whereinthe particles of a) and b) are smaller than 200 μm and whereincomponents a) and b) are used in amounts of from 0.75 to 1.25equivalents of active hydrogen bound to amino nitrogen atoms ofcomponent b) per epoxy equivalent of component a), whereby saidcomposition is applied on a heat-sensitive substrate and cured with theapplication of heat and pressure.
 2. A process according to claim 1,wherein a composition is used, wherein component a) is an aromatic orN-heterocyclic epoxy resin.
 3. A process according to claim 1, wherein acomposition is used, wherein component a) is a bisphenol diglycidylether or a triglycidyl isocyanurate.
 4. A process according to claim 1,wherein a composition is used, wherein component a) is a polyglycidylether of bisphenol A, tetrabromobisphenol A, bisphenol F or of anovolak, or triglycidyl isocyanurate.
 5. A process according to claim 1,wherein a composition is used, wherein component b) is an aminogroup-containing adduct.
 6. A process according to claim 1, wherein acomposition is used, wherein component b) is an amino group-containingadduct of a diglycidyl ether compound of bisphenol A andethylenediamine, diethylenetriamine, 1,2- or 1,4-diaminocyclohexane orisophoronediamine.
 7. A process according to claim 1, wherein acomposition is used, comprising as reinforcing material wood chips orfibres of natural materials.